Fading Vs. Non-Fading Black Horses

Black
horses are the result of a recessive color caused by two gene combinations.
To be black, a black horse must have at least one copy of the dominant gene
for black melanin (known as big "E"). He must also have two copies of the
recessive non-agouti gene (known as little “a”). The presence of at least 1
agouti gene causes the black coloration to be restricted to the mane, tail,
and body points resulting in a bay horse. Therefore, to be black you don’t
want the agouti gene (known as big "A") to be present.

Within the color black, there are various shades. The sooty modifier adds
black hairs throughout the body. This modifier is responsible for making bay
horses “look” like they are black. A black horse with the sooty modifier may
appear blacker throughout.

The silver modifier (not present in the Spanish horse breed as far as we
know), lightens the mane and tail and body – resulting in a brown chocolate
horse with flaxen mane and tail. There are other modifiers as well. The
crème gene (dilution) added to black results in a smokey black – although it
is difficult to distinguish a smokey black from a true black since the
dilution does not really affect black hairs. The roan gene turns a black
horse into a blue roan. The dun gene makes a black horse a grullo. The mealy
effect gene causes a lighter shade on the muzzle, flank and inner thigh and
results in a seal brown horse (actually a type of black). And of course, the
grey modifier gene turns a black horse to grey or white.

Most black horses will experience fading over the summer season. Fading
black horses get reddish-tinged hairs or brown burned areas from sweat or
sun. They may also fade due to poor nutrition. To bring out their best
color, fading blacks must be kept out of the sun or blanketed so that their
hairs are not bleached.

Fading black horses are still blacks, even though during certain times
they may look dark brown. Black horses are born a smoky color, or sometimes
dark bay or brown.

Non-fading black is a blue-black shade that does not fade in the sun. It
is called blue black, raven black, coal black, or jet black. Genetically,
this horse cannot be differentiated from a fading black horse. Non-fading or
jet black horses are common in the Friesian and Arabian horse breed.

Fading black horses are a common form of black in the Andalusian horse
breed. Bay blacks (or bay horses with the sooty modifier) are also common.
Jet black Andalusians are more rare.